OTDR is Optical Time Domain Reflectometer. By emitting a light pulse into the fiber and receiving returned curve information, link condition is detected. When light pulse propagates within an optical fiber, scattering and reflection are generated due to property of the fiber itself, connector, fusion point, bent or other similar events. In OTDR, there are mainly Rayleigh scattering and Finel reflection, some of which will return to OTDR. The following formula shows how OTDR measures distance:d=(c×t)/2(IOR),
where d is distance (fiber length) to be measured, c is velocity of light in vacuum, IOR is index of refraction of the optical fiber, and t is total time from sending signal to receiving the same (round-trip).
Optical cable monitoring system integrates OTDR, optical switch, light source, optical power meter, optical coupler and other hardware devices, applies typical network and database technology, which help the operator complete the system running task easily, view a series of test results. The system automatically manages OTDR equipment, monitors optical cable network, provides feedback on real-time alarm analysis and optical cable performance degradation analysis. All optical cable test data and results are stored in the database for query and analysis.
The online monitoring model is the most commonly used mode of cable monitoring, that is, optical cable monitoring system and business operation system are running at the same time, detection light and business light emitted from the OTDR are coupled and transmitted in the line together. Main function of optical cable monitoring is to detect the line in which optical fiber is in fault, provide accurate fault location information, so that maintenance personnel can go to the location for maintenance. The past SDH networks are single wave system with low outgoing optical power. Now wave division system has been popular with high outgoing optical power, which cannot be completely isolated, resulting jitter in reflected light collected by the OTDR. For the single wave system, SNR is raised usually by increasing the dynamic range, increasing the number of averaging steps. On the other hand, in wavelength division system, the time of analysis is extended if the dynamic range and the number of averaging step is increased continuously, resulting nonlinear effect, and it needs expensive hardware support. Therefore, for the on-line monitoring mode, it is a economical and practical way to design an optimized algorithm to analyze breakpoint of the optical fiber.
There are many patent literatures involved in locating end events. U.S. Pat. No. 5,442,434 uses template matching method to match the end reflection events, which need to test the fiber to build a matching template, and then match reflection events by the template. In process of template matching, it is needed to match each point one by one to calculate matching degree, and design different templates for every curve. The method is time-consuming, requires large memory space. It is difficult to establish different templates, and the overall implementation is complex. Further, in the literature, it is only mentioned reflection end template matching method, without considering case of the non reflection end.
The European patent application EP0468412 uses the difference method to perform data transform on the OTDR curve, which increases the amplitude of the reflection peak. Threshold judgment rule is set by the characteristic of the end of the transformed curve to locate the reflection end event and the non reflection end event. Although this method considers two cases of the end event, such transformation makes the noise amplitude large, even larger than end reflection amplitude. Since the end event is near tail noise, obviously, the method is likely to produce false judgment on the end.
U.S. Pat. No. 6,674,518 determine breakpoint of optical fiber by using a first order differential method and filtering method. This method only considers case of non reflection end, in which after the differential method, breakpoint location will be subjected to mutation, reflection point will also be subjected to mutation. A filter is designed to filter reflection mutation point to locate end mutation position. However, online noise will produce a large amplitude of mutation, the filter cannot completely filtered the same, so this method cannot resist impact of noise.
From above, general method of OTDR detecting end events cannot effectively remove noise, distinguish various end events, and detect a variety of end event in online case, which affects reliability of optical fiber monitoring system. Therefore, a new detection method is needed, which can locate a variety of end events accurately, is simple to implement, and does not need to spend a lot of time and memory space for practical use.